1. Field of the Invention
The present invention relates to a fuel cell stack for mounting in a vehicle. In this fuel cell stack a plurality of fuel cell units, each of which is formed by placing a solid polymer electrolyte membrane between an anode side electrode and a cathode side electrode, are stacked in the horizontal direction with separators placed therebetween. In particular, the present invention relates to a fuel cell stack having excellent vibration resistance and impact resistance.
2. Description of the Related Art
A solid polymer electrolyte fuel cell is formed, for example, by interposing between separators fuel cell units formed by providing an anode electrode on one side and a cathode electrode on the other side of an electrolyte membrane comprising a polymer ion exchange membrane (a cation exchange membrane). This solid polymer electrolyte fuel cell is normally used as a fuel cell stack by arranging a predetermined number of fuel cell units and separators in a stack.
In this type of fuel cell stack, fuel gas, for example, hydrogen gas supplied to the anode side electrode is hydrogen ionized on a catalytic electrode and moves towards the cathode side electrode via the electrolyte membrane that has been humidified to an appropriate degree. The electrons generated in the electrochemical reaction flow through an external circuit and can provide electric energy in the form of a direct current. Because an oxidizing gas such as oxygen gas or air is supplied to the cathode side electrode, the hydrogen ions, the electrons, and the oxygen gas react at the cathode side electrode to generate water.
When the above described fuel cell stack is employed by being mounted in a vehicle, in particular, a passenger vehicle, there are strict limitations on the space that can be taken up by the fuel cell stack in the height direction as a result of the stack commonly being placed under the floor of the vehicle compartment. Accordingly, the height of a fuel cell unit is restricted and a plurality of fuel cell units are stacked in the horizontal direction with separators placed in between each fuel cell unit (see Japanese Unexamined Patent Application, First Publication No. Hei-8-171926 for an example). The fuel cell stack is fixed to vehicle body panel by mounting members provided in end plates at both ends of the fuel cell stack.
An example of this structure can be seen in FIG. 13. In FIG. 13 the numeral 1 indicates a fuel cell stack. In this fuel cell stack 1, a plurality of fuel cell units, each of which is formed by placing a solid polymer electrolyte membrane between an anode side electrode and a cathode side electrode, are stacked in the horizontal direction with separators placed between each. Each fuel cell unit is fastened by a stud bolt 2. A fastening structure portion 3 comprising a coned disc spring or the like is provided at one end in the direction in which the fuel cell stack 1 is stacked, while another fastening structure portion 4 comprising a washer or the like is provided at the other end thereof. These two portions impart the necessary fastening force to each fuel cell unit of the power generating cell portion located in the center portion.
Here, a mounting member 6 used for the installation of the fuel cell stack 1 is mounted on an end plate 5 of the fastening structure member 3 provided at the one end in the stacking direction of the fuel cell units, while a mounting member 8 used for the installation of the fuel cell stack 1 is also mounted on a backup plate 7 of the fastening structure member 4 at the other end of the fuel cell stack 1.
The two mounting members 6 and 8 provided at the two ends of the fuel cell stack 1 are fixed to the vehicle body panel 9.
However, in the conventional fuel cell stack 1 the problem exists that if the stacking length of the stack is made longer due to the increasing number of fuel cell units as a result of attempts to raise the output voltage of the fuel cell stack 1, the natural frequency of the fuel cell stack 1 decreases resulting in deterioration of vibration resistance to vibration generated by repeated starting and stopping of the vehicle or while the vehicle is traveling or the like.
In addition, the problem also exists that if the stacking length of the stacking is made longer, because the distance between the center of gravity of the fuel cell stack 1 and the mounting members 6 and 8 (which are the support points) is lengthened, the load (particularly the moment) acting on the mounting members 6 and 8 provided at both ends of the fuel cell stack 1 increases. This results in the impact resistance when an impact force is applied to the vehicle being reduced.
In response to this, although it may also be possible to employ a greater number of fuel cell units each having a shorter stack length, because the stud bolts that fasten the fuel cell units stacked together via separators, the fastening structure portions, the manifold for supplying the fuel gas and the oxidizing gas and the like, the piping, the bus bars for the electrical connections, and the like are all additionally necessary, the number of parts as well as the number of assembly steps are greatly increased and the problem arises that this tends to cause an increase in the weight of the vehicle as well as in the space occupied by the fuel cell stack.